An image forming apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus forms an image by developing an electrostatic latent image formed on a photoconductor with toners, transferring the developed toner image to a recording medium such as paper, and then fixing the toner image on the medium by heating. In the formation of a full-color image, generally, four colors of toners, namely, black, yellow, magenta, and cyan are used in development. After toner images of the respective colors are transferred to a recording medium and overlaid together, they are fixed on the medium by heating at the same time.
In order to reduce environmental impacts to the earth, toners are further required to have low-temperature fixability. If the softening characteristics of the toner are reformed to be set at a lower temperature in order to improve the low-temperature fixability, a problem occurs that the heat resistance storage stability of the toner is degraded. Degradation of the heat resistance storage stability of toner is a problem that the toner is solidified and cannot preserve its inherent flowability, when it has returned to room temperature after it melted under high-temperature, high-humidity conditions. Further, melting adhesion (hot offset) of a small amount of toner to the fixing member, which is likely to occur around the upper limit of the range of fixing temperatures, is more likely to occur. It has been difficult for the conventional toner to satisfy the low-temperature fixability and the heat-resistance storage stability at the same time.
Furthermore, if the softening characteristics of the toner are reformed to be set at a lower temperature, the developing stability of the toner is degraded. That is, the toner softens due to stirring stress in the development, and adheres to the developing member. It has also been difficult to overcome this problem at the same time as satisfying the above demands.
Meanwhile, it is known to use a crystalline resin as a binder resin of the toner for softening the toner (PTL 1). That is, a crystalline resin can rapidly soften at the melting point of the resin, which suggests that it might be possible to lower the softening temperature of the toner to around the melting point of the resin while securing the heat-resistance storage stability at equal to or lower than the melting point. However, it is actually very difficult to control the viscoelasticity at low temperatures. It is therefore very difficult to satisfy low-temperature fixability, heat resistance storage stability of the toner, hot offset resistance, and developing stability at the same time at high levels.